As a conventional common liquid crystal display apparatus, as shown in FIG. 15, one comprising a polarizing plate 102A on the incident side, a polarizing plate 102B on the output side, and a liquid crystal cell 104 can be presented. The polarizing plates 102A and 102B are constituted so that only a linearly polarized light (it is shown schematically by an arrow in the figure) having a vibration surface in a predetermined vibration direction is selectively transmitted, and they are provided facing to each other in the crossed Nicol state so that each of their vibration direction is perpendicular with each other. Moreover, the liquid crystal cell 109 comprises a large number of cells corresponding to the pixels, and is disposed in between the polarizing plates 102A and 102B.
Here, in such liquid crystal display apparatus 100, a case of employing VA (vertical alignment) system, wherein a nematic liquid crystal having a negative dielectric anisotropy is sealed in the liquid crystal cell 104 (in the figure, the liquid crystal director is shown schematically with dot lines), will be presented for an example. When the linearly polarized light, which has been transmitted through the incident side polarizing plate 102A, is transmitted through a portion of non-driven state cell, among the liquid crystal cell 104, its phase is not shifted, so as to be blocked by the output side polarizing plate 102B. On the other hand, when it is transmitted through a portion of the driven state cell, among the liquid crystal cell 104, the phase of the linearly polarized light is shifted so that a quantity of light corresponding to the phase shift amount is transmitted through the output side polarizing plate 102B, so as to be output. Thereby, by optionally controlling the driving voltage of the liquid crystal cell 104 per each cell, a desired image can be displayed on the output side polarizing plate 102B side. The liquid crystal display apparatus 100 is not limited to those having the above-mentioned aspect of the light transmission and blockage. A liquid crystal display apparatus having the configuration in which the output light from a portion of the non-driven state cell, among the liquid crystal cell 104, is transmitted through the output side polarizing plate 102B so as to be output, whereas the output light from a portion of the driven state cell is blocked by the output side polarizing plate 102B has also been proposed.
Considering the case in which the linearly polarized light is transmitted through a portion of the non-driven state cell, among the above-mentioned VA system liquid crystal cell 104, the liquid crystal cell 104 has a birefringence so that the refractive index in the thickness direction and the refractive index in the plane direction differ with each other. Therefore, among the linearly polarized light transmitted through the incident side polarizing plate 102A, the incident light along the normal line of the liquid crystal cell 104 is transmitted through the same without being its phase shifted. However, among the linearly polarized light transmitted through the incident side polarizing plate 102A, the phase of the incident light in the direction inclined with respect to the normal line of the liquid crystal cell 104 is shifted when transmitting the liquid crystal cell 104, so as to be elliptically polarized. This phenomenon derives from the function of the liquid crystal molecules, aligned in the vertical direction in the liquid crystal cell 104, as a positive C plate. The size of the generated phase difference with respect to the light (transmitted light) transmitted through the liquid crystal cell 104 is influenced by the birefringence value of the liquid crystal molecules sealed in the liquid crystal cell 104, the thickness of the liquid crystal cell 104, the wavelength of the transmitted light, or the like.
Due to the above-mentioned phenomenon, even when a cell in the liquid crystal cell 104 is in the non-driven state so that a linearly polarized light is inherently to be transmitted as it is and blocked by the output side polarizing plate 102B, a part of the output light, which is in a direction inclined with respect to the normal line of the liquid crystal cell 104, is leaked from the output side polarizing plate 102B.
Therefore, in the above-mentioned conventional liquid crystal display apparatus 100, a problem is involved in that the display quality of an image observed from a direction inclined with respect to the normal line of the liquid crystal cell 104 is deteriorated mainly due to the contrast deterioration, compared with an image observed form the front side (problem of viewing angle dependency).
In order to improve the problem of viewing angle dependency in the above-mentioned conventional liquid crystal display apparatus 100, various techniques have been developed so far. As one of them, for example as it is disclosed in Japanese Patent Application Laid Open (JP-A) Nos. 3-67219 or 4-322223, a liquid crystal display apparatus, in which the optical compensation is carried out by using a retardation layer (retardation layer showing the birefringence) having a cholesteric regularity molecular structure and disposing such retardation layer in between a liquid crystal cell and a polarizing plate, is known.
Here, in a retardation optical element having a cholesteric regularity molecular structure, a selective reflected wavelength represented by λ=nav·p (p: helical pitch in the helical structure of the liquid crystal molecules, nav: average refractive index in the plane orthogonal to the helical axis) is adjusted, for example as disclosed in JP-A Nos. 3-67219 or 4-322223, so as to be smaller than or larger than the wavelength of the transmitted light.
In the retardation optical element as mentioned above, as in the case of the above-mentioned liquid crystal cell, the phase of the incident linearly polarized light, in a direction inclined with respect to the normal line of the retardation layer, is shifted when transmitting the retardation layer so as to be elliptically polarized. This phenomenon derives from the function of the cholesteric regularity molecular structure as a negative C plate. The size of the phase difference generated with respect to the transmitted light through the retardation layer (transmitted light) is influenced by the birefringence value of the liquid crystal molecules in the retardation layer, the thickness of the retardation layer, the wavelength of the transmitted light, or the like.
Therefore, by use of the above-mentioned retardation optical element, by optionally designing the retardation layer such that the phase difference generated by the liquid crystal cell of the VA system, which acts as a positive C plate, and the phase difference generated in the retardation layer, which acts as a negative C plate, can be offset, the problem of the viewing angle dependency of the liquid crystal display apparatus can dramatically be improved.
On the other hand, in JP-A No. 7-175065, a liquid crystal display comprising a large number of micro domains, of about several μm to several tens μm diameter, that are not fixed is disclosed as a liquid crystal cell of the TN (twisted nematic) system. A chiral nematic liquid crystal layer used as a TN liquid crystal is designed so as to function as an optical rotation layer, not to function as a retardation layer. Therefore, the twist angle of the above-mentioned TN liquid crystal is designed such that the twist angles of the above-mentioned large number of micro domains are same in a range of 0 degree to about 270 degrees (0 to 0.75 pitch when converted to the chiral pitch). In, the case the chiral pitch of the TN liquid crystal is presumed to be 1 or more pitches, the selective reflected wavelength of the TN liquid crystal is longer than the wavelength of an incident visible light.
In SID '93 Digest, 622 (1993), R. Holding et al. discloses, similarly as a liquid crystal cell forming method of the TN system, a liquid crystal display comprising a liquid crystal layer, in an amorphous state, which is not fixed. A chiral nematic liquid crystal layer used as a TN liquid crystal is designed so as to function as an optical rotation layer, not to function as a retardation layer. Therefore, the twist angle of the above-mentioned TN liquid crystal is designed to be 90 degrees (0.5 pitch when converted to the chiral pitch). The TN system includes a normally black mode in which a TN cell is interposed between two polarizing plates with the absorption axes provided parallel, and a normally white mode in which a TN cell is interposed between polarizing plates cross Nicol. According to SID '93 Digest, 622 (1993) by R. Holding et al., when TN liquid crystal display comprising a liquid crystal layer, in an amorphous state, which is not fixed is made into a normally black mode, the transmission is as much as 3% so as to induce the contrast deterioration. In the case the chiral pitch of the TN liquid crystal is presumed to be 1 or more pitches, the selective reflected wavelength of the TN liquid crystal is longer than the wavelength of an incident visible light.
In also SID '94 Digest, 915 (1994), Y. Iimura et al. disclose, similarly as a liquid crystal cell forming method of the TN system, a liquid crystal display comprising a liquid crystal layer in an amorphous state. The above-mentioned liquid crystal layer in the amorphous state has a 10 to 100 μm brush width, with several domains present therebetween, and the directors of the adjacent domains are substantially continuous. A chiral nematic liquid crystal layer used as a TN liquid crystal is designed so as to function as an optical rotation layer, not to function as a retardation layer. Therefore, the twist angle of the above-mentioned TN liquid crystal is designed to be 90 degrees (0.5 pitch when converted to the chiral pitch). In the case the chiral pitch of the TN liquid crystal is presumed to be 1 or more pitches, the selective reflected wavelength of the TN liquid crystal is longer than the wavelength of an incident visible light.
Moreover, JP-A No. 2002-258053 discloses a mono-domain circularly polarized light extracting optical element in which directors of liquid crystal molecules, in the entire range of the liquid crystal layer surface having cholesteric regularity, are coincident. Thereby, the bright and dark pattern, observed at the time of interposing the cholesteric liquid crystals between the polarizing plates in a cross Nicol state, is dissolved.
However, in the case the above-mentioned retardation optical element (retardation layer having the molecular structure of the cholesteric regularity) is disposed between the liquid crystal cell and the polarizing plate, even though the problem of viewing angle dependency can be improved, there is a problem that a bright and dark pattern is generated in the displayed image and that the display quality is extremely deteriorated, except the case of JP-A No. 2002-258053.